How do i determine the resistance of the primary side of a AC transformer and the secondary?

Question

this is a question i came across while troubleshooting an aircondtioning unit i suspeted that the resistance was wrong and creating a short in the circuit. my question then is really how would i figure out the corect resistance based on the information on the tag which was

primary 460VAC and a rating of 500 VA

Answer 1

DC resistance? use an ohmmeter.

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Answer 2

If you're talking about the copper (coil) resistance, just use an ohmmeter across the contacts of the primary/secondary coils.  Due to the inductive load, the value may take a moment so stabilize.

Answer 3

when troubleshooting ac don't get yourself involved with resistance or dc measurements. measure the primary and secondary ac currents, the transformer winding is not shorted if primary 500VA is equal to secondary VA (assuming 100 efficiency). for safety reason test the performance of the transformer using lower voltage as suggested by tinkerta.

if the transformer is short circuited, the transformer will be overheated and also it will not delivered the secondary voltage as shown on the name tag.

Answer 4

I would only be guessing to try to determine the DC resistance from that information, but I'd think probably rather less than 50Ω for the primary, just as a guess maybe 25Ω that would result in about 5% copper losses at 500VA @460V for the primary, which seems plausible for a small transformer, but I'm not a transformer expert, and as I said it is a guess.

If I was testing for a short, I'd first check if the resistance was very close to zero in which case it is shorted, then I think I would completely disconnect the transformer and then power it from 120V AC (since I'm in the US) with a 100W or so incandescent light bulb in series to limit current if the transformer is shorted. With no load on the secondary, the lamp should not light with any significant brightness, it probably won't light at all, but there might be a dim glow due to magnetic losses, However if the transformer is shorted (either primary or secondary) the lamp will glow.